Ceramic discharge lamp having metal end cap

ABSTRACT

An electric discharge lamp comprising a tubular lighttransmitting envelope of alumina ceramic closed at each end by refractory metal end caps. The envelope contains a discharge sustaining filling and electrodes supported by the end caps. At least one end cap is a thin-walled metal thimble having a skirt portion sealed to the end of the tube and a central blind teat in which the electrode shank is seized. The need for critical welding or brazing to the end cap is thereby avoided.

United States Patent [191 McVey et all CERAWC DISCHARGE LAW HAVING METAL END CAP [75] Inventors: (Iharles l. McVey, Shaker Hts;

ByronR. Collins, Euclid, both of Ohio [73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Sept. 25, 1972 [21] Appl. No.: 291,804

[52] US. Cl 313/217, 313/220, 313/252, 313/284 [51] lint. Cl 11-11011 1/94, HOlj 19/46 [58] Field of Search 313/217, 184, 220, 284, 313/252-253 [56] References (Iited UNITED STATES PATENTS 3,248 590 4/1966 Schmidt 313/184 Aug. 27, 1974 1476,96) 11/1969 Ennulat et a1. 313/217 3,708,710 1/1973 Smyser et a1. 3,716,744 2/1973 Delcmbre et a1. 313/220 Primary ExaminerJames W. Lawrence Assistant ExaminerMarvin Nussbaum Attorney, Agent, or Firm-Ernest W. Legree; Henry P. Truesdell; Frank L. Neuhauser ABSTRACT An electric discharge lamp comprising a tubular lighttransmitting envelope of alumina ceramic closed at each end by refractory metal end caps. The envelope contains a discharge sustaining filling and electrodes supported by the end caps. At least one end cap is a thin-walled metal thimble having a skirt portion sealed to the end of the tube and a central blind teat in which the electrode shank is seized. The need for critical welding or brazing to the end cap is thereby avoided.

7 Claims, 2 Drawing Figures CERAMIC DISCHARGE LAMP HAVING METAL END CAP I BACKGROUND OF THE INVENTION The invention relates to high intensity metal vapor arc lamps utilizing a tubular ceramic envelope, and is particularly useful with high intensity sodium vapor lamps of the kind described in U.S. Pat. No. 3,248,590 Schmidt, entitled High Pressure Sodium Vapor Lamp."

Such lamps utilize an elongated inner envelope or tube of ceramic material resistant to the attack of sodium at high temperatures, such as high density poly crystalline alumina, for containing the arc discharge. The filling comprises an amalgam of sodium and mercury and a rare gas, preferably xenon, to facilitate starting. The ends of the alumina tube are sealed by refractory metal closure members which carry the electrodes between which the discharge passes in operation. Niobium is preferred for the end caps because it has a coefficient of expansion closely matching that of the alumina ceramic. One well-known construction utilizes an end cap of niobium having an expanded skirt portion which fits around the end of the alumina tube and is bonded thereto by a thin annular layer of glassy sealing material comprising aluminum oxide and calcium oxide optionally with modifiers. Each end cap supports an electrode extending along the axis of the tube and serves as an electrical inlead. The electrode comprises a tungsten rod supported from the end cap and having a double coil of tungsten wound around its inner end. The electrode is activated by a quantity of low work function material consisting of metal oxides retained in the interstices of the coil.

The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with the usual screw base to which the end caps are electrically connected. The interenvelope space is usually evacuated in order to conserve heat and maintain the arc tube at a sufficiently high temperature.

SUMMARY OF THE INVENTION In making the lamp, it is necessary to exhaust the air therefrom and introduce the filling consisting of sodium, mercury, and xenon gas and this has been done using an exhaust tube extending through one of the end caps. The practice which has been commonly adopted is to use niobium end caps and to provide each end cap with a niobium tube extending through it. The niobium tube of one end cap has an opening or vent into the lamp envelope and serves as an exhaust port during manufacture. The niobium tube of the other end cap has no opening into the envelope and may be referred to as a dummy. The electrodes have been supported by inserting their shanks into the inner ends of these niobium tubes which are then crimped. In the finished lamp, under proper orientation the vented exhaust tube is located lowermost and serves as a reservoir for sodium-mercury amalgam; the dummy exhaust tube is located at the upper end. This construction has certain advantages resulting from the symmetry at both ends. However it is expensive due to the quantity of niobium used and the electron beam welding required to assure hermetic closures. In electron beam welding, an elaborate technique must be used wherein the parts must be placed in an evacuable chamber. The object of the invention is to simplify the construction and reduce its cost while retaining symmetry as much as possible.

In accordance with our invention we eliminate the dummy exhaust tube and provide for the corresponding end cap an imperforate thin-walled thimble of metal which will seal to the ceramic, suitably of niobium. The thimble has a skirt portion sealed to the end of the ceramic tube and a central blind teat in which the electrode shank is seized. This eliminates the dummy niobium tube and avoids the need for hermetically welding it through the end cap. Also if the niobium tube should be an open one, the need for hermetically closing it is avoided. A common practice has been to attach a strap to the dummy exhaust tube to support the arc tube within the outer jacket. Such strap may be attached to the projecting teat in the same fashion so that no change need be made in the simple mounting arrangement.

DESCRIPTION OF DRAWING In the drawing:

FIG. 1 is a front view of a high pressure sodium vapor lamp embodying the invention.

FIG. 2 is a sectional view of the upper end of the arc tube to a larger scale showing the improved end cap.

DESCRIPTION OF PREFERRED EMBODIMENT A high intensity sodium vapor discharge lamp 1 in which the invention is embodied is illustrated in FIG. 1 and comprises an outer vitreous envelope or jacket 2 of elongated ovoid shape. The neck 3 of the jacket is closed by a re-entrant stem 4 having a press 5 through which extend relatively stiff inlead wires 6, 7 connected at their outer ends to the threaded shell 8 and insulated center contact 9 of a conventional mogul screw base.

The inner envelope or arc tube 11 is made of sintered high density polycrystalline alumina ceramic per U.S.

Pat. No. 3,026,210 Coble, Transparent Alumina and Method of Preparation, or of other lighttransmitting ceramic capable of withstanding the attack of sodium vapor at high temperatures. The ends of the slender tube are closed by thimble-like niobium metal end caps 12, 13. Each comprises an end portion and a skirt portion which is sealed to the alumina by means of a sealing composition comprising a major proportion of aluminum oxide and calcium oxide and minor proportions of magnesium oxide and barium oxide. The sealing material is shown exaggerated in thickness at 14 in FIG. 2.

Thermionic electrodes 15, 16 are mounted in the ends of the arc tube. Each electrode comprises an inner tungsten wire coil 17 which is wound over a tungsten shank 18 extending from the end cap and an outer tungsten wire coil 19 screwed over the inner coil. The electrodes are activated by metal oxides retained in the interstices between turns of the coil, a preferred material being dibarium calcium tungstate Ba CaWO as described and claimed in copending U.S. Pat. application Ser. No. 97,907, filed Dec. 14, 1970 by William E. Smyser et al., entitled, Discharge Lamp Thermionic Cathode Containing Emission Material, and assigned to the same assignee as this invention, now U.S. Pat. No. 3,708,710. The emitting portion of the electrode comprises the shank tip and the coils 17 and 19 with the metal oxides thereon. A disc or shield 20 is mounted on the shank to the rear of the electrode and prevents the are from striking at the inner boundary 21 between the niobium end cap and a glassy sealing material 14.

End cap 12 at the lower end of the ceramic tube has its end portion pierced through and has a niobium tube 22 extending therethrough. Tube 22 is pierced through at 23 and is used as an exhaust tube during manufacture. After the xenon gas filling and the sodium mercury amalgam have been introduced into the arc tube, exhaust tube 22 is hermetically pinched off by a cold weld indicated at 24 and serves thereafter as a reservoir for condensed sodium-mercury amalgam. The shank 18 of the lower electrode is inserted into the inner end of the exhaust tube which is crimped around it.

In accordance with our invention, upper end cap 13 best seen in FIG. 2 does not have an exhaust tube extending through it. It is a thin'walled niobium thimble having an end portion 25 and a skirt portion 26 sealed to the end of the ceramic tube. End portion 25 has a central blind teat 27 in which electrode shank 18 is inserted and seized. The shank may be locked in place simply by mechanically crimping the teat, or alternatively by brazing or by resistance welding. The need for a connecting tube such as a dummy exhaust tube between the tungsten electrode shank and the niobium end cap is thus eliminated. At the place where the dummy exhaust tube of niobium passed through the end cap, the electron beam weld necessary to assure a hermetic joint is likewise eliminated. Also yttrium brazing sometimes necessary to seal and cover the exposed end of the electrode shank within the dummy exhaust tube, or inert gas arc welding of the electrode shank used as an alternative, are no longer needed.

The are tube is supported within the outer envelope by means of a mount comprising a single side rod 28 which extends the length of the envelope from inlead 6 at the stem end to a dimple 29 at the dome end to which it is anchored by a resilient clamp 30. End cap 12 at the lower end of the arc tube is connected to the frame by welded expansion strap 31 and band 32. End cap 13 at the upper end is connected to inlead 7 through band 33 and connecting rod 34. Band 33 may be resistance welded to teat 27 in the same fashion as it had been welded to the projecting end of the dummy exhaust tube formerly used. Thus no change need be made in the mounting arrangement for supporting the arc tube within the outer jacket.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A ceramic discharge lamp comprising an elongated light-transmitting ceramic envelope, a pair of end caps sealed to the ends of said envelope, electrodes supported by said end caps at opposite ends of said envelope, a discharge sustaining filling within said envelope, one of said end caps being a thin-walled thimble of metal having an end portion and a skirt portion sealed around the end of said ceramic envelope, said end portion having a central part of substantial area which is generally flat except for a blind teat extending outwardly therefrom, one of said electrodes comprising an emitting portion mounted on a metal shank near one end, the other end of said shank being inserted and seized in the teat of said one end cap.

2. A lamp as in claim 1 wherein said ceramic is alumina and said end cap is niobium.

3. A lamp as in claim 1 wherein said ceramic is alumina, said end cap is niobium and the shank of said one electrode is tungsten.

4. A ceramic discharge lamp comprising an elongated light-transmitting ceramic envelope, a pair of end caps sealed to the ends of said envelope, electrodes supported by said end caps at opposite ends of said envelope, a discharge sustaining filling within said envelope, said end caps being thinwalled thimbles of metal having an end portion and a skirt portion sealed around the end of said ceramic envelope, the end portion of one end cap having an exhaust tube extending therethrough, the end portion of the other end cap having a central part of substantial area which is generally flat except for a blind teat extending outwardly therefrom, said electrodes comprising a tungsten coil wound around a tungsten shank, the shank of one electrode being inserted and seized in the exhaust tube of one end cap and the shank of the other electrode being inserted and seized in the teat of the other end cap.

5. A lamp as in claim 4 wherein said ceramic is alumina and said end caps are niobium.

6. A lamp as in claim 4 wherein said ceramic is alumina, said end caps are niobium and said filling comprises a sodium mercury amalgam and an inert starting gas.

7. A lamp as in claim 4 wherein said ceramic is alumina, said end caps are niobium, and which includes a vitreous outer jacket surrounding said ceramic envelope and a mount supporting said ceramic envelope within said outer jacket comprising metal straps, one attached to the exhaust tube of said one end cap, and

the other attached to the teat of said other end cap. 

1. A ceramic discharge lamp comprising an elongated lighttransmitting ceramic envelope, a pair of end caps sealed to the ends of said envelope, electrodes supported by said end caps at opposite ends of said envelope, a discharge sustaining filling within said envelope, one of said end caps being a thin-walled thimble of metal having an end portion and a skirt portion sealed around the end of said ceramic envelope, said end portion having a central part of substantial area which is generally flat except for a blind teat extending outwardly therefrom, one of said electrodes comprising an emitting portion mounted on a metal shank near one end, the other end of said shank being inserted and seized in the teat of said one end cap.
 2. A lamp as in claim 1 wherein said ceramic is alumina and said end cap is niobium.
 3. A lamp as in claim 1 wherein said ceramic is alumina, said end cap is niobium and the shank of said one electrode is tungsten.
 4. A ceramic discharge lamp comprising an elongated light-transmitting ceramic envelope, a pair of end caps sealed to the ends of said envelope, electrodes supported by said end caps at opposite ends of said envelope, a discharge sustaining filling within said envelope, said end caps being thinwalled thimbles of metal having an end portion and a skirt portion sealed around the end of said ceramic envelope, the end portion of one end cap having an exhaust tube extending therethrough, the end portion of the other end cap having a central part of substantial area which is generally flat except for a blind teat extending outwardly therefrom, said electrodes comprising a tungsten coil wound around a tungsten shank, the shank of one electrode being inserted and Seized in the exhaust tube of one end cap and the shank of the other electrode being inserted and seized in the teat of the other end cap.
 5. A lamp as in claim 4 wherein said ceramic is alumina and said end caps are niobium.
 6. A lamp as in claim 4 wherein said ceramic is alumina, said end caps are niobium and said filling comprises a sodium mercury amalgam and an inert starting gas.
 7. A lamp as in claim 4 wherein said ceramic is alumina, said end caps are niobium, and which includes a vitreous outer jacket surrounding said ceramic envelope and a mount supporting said ceramic envelope within said outer jacket comprising metal straps, one attached to the exhaust tube of said one end cap, and the other attached to the teat of said other end cap. 